This invention relates to an improved process for the preparation of arbutin or its derivatives which are useful as raw materials for industrial chemicals, medicines and cosmetics. More particularly, the invention relates to an efficient process for preparing tetrahydropyran derivatives on an industrial scale.
A variety of methods have been known as processes for the preparation of 4-hydroxyphenyl-xcex2-D-glucopyranoside represented by the following formula, i.e., arbutin. 
These known methods require a treatment to remove acyl groups by hydrolysis in an aqueous solvent from an intermediate wherein hydroxyl groups in the glucopyranoside are acylated. This hydrolysis is carried out in the presence of a base such as sodium hydroxide, potassium hydroxide and sodium methylate. In this process, any inorganic cation forming during the hydrolysis may contaminate the object product. Japanese Patent Kokai 62-226974 proposes a method of treating the reaction solution after hydrolysis with a cation exchange resin to avoid the contamination.
However, a method for preparing arbutin or its derivatives by using a cation exchange resin on an industrial scale has the following problems.
i) Increase in number of ion-exchange columns and peripheral equipments requires more installation places, resulting in increased cost of the equipments.
ii) Regeneration treatment of ion exchange resins is time-consuming.
iii) Much volume of washing water used for the regeneration brings about increased volume of drainage.
In the circumstances, there is still a demand for an efficient process for the preparation of arbutin or its derivatives on an industrial scale.
As a result of studying the above-mentioned problems, the present inventors have found that the tetrahydropyran derivatives of the following formula (II) can be easily prepared on an industrial scale by solvolyzing in an organic solvent a compound of the following formula (I) in the presence of a base, followed by neutralizing with an acid, and crystallizing.
Thus the present invention provides a process for the preparation of a tetrahydropyran derivative of formula (II) 
wherein Z1, Z2, Z3, Z4 and Z5 have respectively the same meaning as defined below for R1, R2, R3, R4 and R5, provided that a C1-C4 alkylcarbonyloxy group in formula (I) is converted into a hydroxyl group, and a C1-C4 alkylcarbonyloxy C1-C4 alkyl group is converted into a hydroxyl C1-C4 alkyl group, which comprises solvolyzing in an organic solvent a compound of formula (I) 
wherein R1, R2, R3 and R4 may be the same or different, and each represents a hydrogen atom, a hydroxyl group, a C1-C4 alkoxy group, a C1-C4 alkylcarbonyloxy group or an aryl C1-C4 alkyloxy group, and R5 represents a hydrogen atom, a C1-C4 alkyl group, a hydroxy C1-C4 alkyl group or a C1-C4 alkylcarbonyloxy C1-C4 alkyl group, provided that at least one of R1, R2, R3 and R4 is a C1-C4 alkylcarbonyloxy group, in the presence of a base, followed by neutralizing with an acid and crystallizing.
In formulae (I) and (II),
a C1-C4 alkyl group includes methyl, ethyl, propyl, isopropyl, butyl, sec-butyl and tert-butyl;
a C1-C4 alkoxy group includes methoxy, ethoxy, propoxy, isopropoxy, butoxy, sec-butoxy and tert-butoxy;
a C1-C4 alkylcarbonyloxy group includes acetyloxy, propionyloxy, butyryloxy, isobutyryloxy, valeryloxy, isovaleryloxy group and the like;
an aryl C1-C4 alkyloxy group includes a phenyl C1-C4 alkyloxy, e.g., benzyloxy, phenethyloxy and the like;
a hydroxy C1-C4 alkyl group includes hydroxymethyl, 2-hydroxyethyl, 3-hydroxypropyl, 2-hydroxy-1-methylethyl, 4-hydroxybutyl and the like; and
a C1-C4 alkylcarbonyloxy C1-C4 alkyl group includes acetyloxymethyl, 2-acetyloxyethyl, 3-acetyloxypropyl, 4-acetyloxybutyl, propionyloxymethyl, 2-propionyloxyethyl, 3-propionyloxypropyl, 4-propionyloxybutyl, butyryloxymethyl, 2-butyryloxyethyl, 3-butyryloxypropyl, isobutyryoxymethyl, 2-isobutyryoxyethyl, valeryloxymethyl, 2-valeryloxyethyl, 3-valeryloxypropyl and the like.
Illustrative examples of compounds of formula (I) include 4-acetyloxyphenyl-2,3,4,6-tetra-O-acetyl-xcex2-D-glucopyranoside, 4-hydroxyphenyl-2,3,4,6-tetra-O-acetyl-xcex2-D-glucopyranoside, 4-methoxyphenyl-2,3,4,6-tetra-O-acetyl-xcex2-D-glucopyranoside, 4-benzyloxyphenyl-2,3,4,6-tetra-O-acetyl-xcex2-D-glucopyranoside, 4-acetyloxyphenyl-2,3,4,6-tetra-O-acetyl-xcex2-D-galactopyranoside, phenyl-2,3,4,6-tetra-O-acetyl-xcex2-D-glucopyranoside, phenyl-2,3,4,6-tetra-O-acetyl-xcex2-D-galactopyranoside and the like.
Illustrative examples of compounds of formula (II) include 4-hydroxyphenyl-xcex2-D-glucopyranoside (arbutin), 4-methoxyphenyl-xcex2-D-glucopyranoside, phenyl-xcex2-D-glucopyranoside, 4-hydroxyphenyl-xcex2-D-galactopyranoside, phenyl-xcex2-D-galactopyranoside, 4-benzyloxyphenyl-xcex2-D-glucopyranoside and the like.
The tetrahydropyran derivatives of formula (II) can be prepared by solvolyzing in an organic solvent the compound of formula (I) in the presence of a base, followed by neutralization and purification by crystallization, with no use of an ion exchange resin which has been used in the prior art. In this case, a concentration of salts forming by neutralization is adjusted to a level at which arbutin or its derivatives are soluble in a crystallization solvent, by appropriately controlling kinds and amounts of bases used in solvolysis and kinds and amounts of acids used in neutralization.
This operation enables the preparation of arbutin or its derivatives in high yield with no use of special purification means such as an ion exchange.
The temperature in solvolysis is in the range from room temperature to the boiling point of a solvent, preferably the boiling point of the solvent used. The time required for solvolysis depends on raw materials, bases, solvents and the like, but is usually within the range from one hour to 24 hours.
Bases used in the present invention include hydroxides such as sodium hydroxide, potassium hydroxide and the like, alcoholates such as sodium methylate, sodium ethylate, potassium t-butylate and the like. An amount of bases used ranges from 1 mol % to 20 mols % based on the compound of formula (I). Acids used for neutralizing bases include organic acids such as formic acid, acetic acid, p-toluenesulfonic acid, methanesulfonic acid and the like. As an organic solvent, alcohols can be used such as methanol, ethanol and the like. The solubility of a salt in each solvent (100 g) is, for example, in the case of sodium acetate, 4.51 g in ethanol at room temperature and 16.00 g in methanol at 15xc2x0 C., and in the case of sodium formate, 3.52 g in methanol at 15xc2x0 C.